Fluid streams having time-variant properties are generated in many processes which operate in cyclic or discontinuous modes. The time variance of certain stream properties can have an undesirable effect on the use of the stream elsewhere in the process or in another process. For example, in the recovery of hydrogen from synthesis gas by pressure swing adsorption, blowdown and purge streams are generated in which the concentrations of combustible components vary periodically with time. These byproduct streams typically are used as fuel to fire a process furnace, and the performance of the furnace can be adversely affected by the resulting time-variant heating value of the fuel stream. Other examples can be found in cyclic or discontinuous processes such as the smelting of ore in the primary metals industries, operation of batch chemical reactors or the venting of process upsets to buffer tanks in the chemical process industries, and variations in wastewater composition in wastewater treatment processes.
Typical time-variant stream properties are temperature, pH, and the concentrations of particular components in the stream. The amplitude of the time variance of the properties of such streams can be reduced or attenuated by two well-known methods.
In one method, something is added to or removed from the stream in a controlled manner to compensate for the variations. For example, heat can be added to or removed from a stream to control temperature or another stream can be added under controlled conditions to the time-variant stream, such as the addition of acid or alkali to control the pH of a stream. This method often is used when the time variance of a desired property is largely random in nature.
In the other method, the time-variant stream is introduced into a holding volume such that the natural capacitance of the volume reduces or attenuates the amplitude of the time variance of the outlet stream properties relative to the inlet stream properties. Nothing is added to or removed from the stream. Mixing of the fluid in the holding volume promotes a constant composition throughout the volume and improves the reduction of the time variance of the outlet stream relative to the inlet stream. This method often is used when the time variance of a desired property is primarily periodic in nature. Mixing can be effected by mechanical agitation, by inlet distributors or jets which utilize pressure drop to promote mixing, or by baffles which promote turbulence.
The appropriate use of liquid or gaseous byproduct streams can have a significant positive impact on the capital and operating costs of a process plant. When these byproduct streams have time-variant properties, mixing of some kind usually is required to reduce the time-variant properties before these byproduct streams are used in the process. Improved mixing methods thus are desirable to reduce initial plant investment and ongoing plant operating cost. The invention disclosed below and defined by the claims which follow offers a new method for attenuating the amplitude of time-variant properties of a fluid stream by controlled mixing in an enclosed volume.
An embodiment of the invention relates to an apparatus for attenuating the amplitude of a time-variant property of a fluid stream which comprises an enclosed volume, wherein the enclosed volume includes
(a) a mixing zone having an inlet and three or more outlets;
(b) piping means for introducing an inlet fluid stream into the inlet of the mixing zone; and
(c) piping means for withdrawing from the mixing zone three or more individual fluid portions through the three or more outlets and combining the three or more individual fluid portions into an outlet fluid stream.
The distance between at least one pair of adjacent outlets may be different than the distance between another pair of adjacent outlets and the distance between adjacent outlets may decrease as the distance of each outlet from the inlet increases.
The apparatus may further comprise piping means for withdrawing a portion of the fluid from the piping means of (b) and introducing it into the outlet fluid stream in the piping means of (c). The mixing zone may comprise a cylindrical volume, the inlet means may be at either end of the cylindrical volume, and the outlets may be arranged in a line parallel to the axis of the cylindrical volume. The outlets may comprise openings of different cross sectional areas.
Another embodiment relates to an apparatus for attenuating the amplitude of a time-variant property of a fluid stream which comprises an enclosed volume, wherein the enclosed volume includes
(a) a mixing zone having an inlet and three or more outlets;
(b) piping means for introducing an inlet fluid stream into the inlet of the mixing zone; and
(c) piping means for withdrawing from the mixing zone three or more individual fluid portions through the three or more outlets and combining the three or more individual fluid portions into an outlet fluid stream.
The distance between at least one pair of adjacent outlets may be different than the distance between another pair of adjacent outlets. The mixing zone may comprise a cylindrical volume which is characterized by an axis, and the axial distance Dn from the inlet to any outlet may be defined by       D    n    =            (              d        /        N            )        ⁢                  ∑                  i          =          0                          n          -          1                    ⁢              xe2x80x83            ⁢              (                  N          -          i                )            
where n is an integer with a value of 1 through N, integers 1 through N denote sequential outlets, N is the total number of outlets from the mixing zone, the outlet denoted by n=1 is the outlet nearest the inlet, the outlet denoted by n=N is the outlet farthest from the inlet, and d is the axial distance between the inlet and the outlet nearest the inlet.
The piping means of (c) may include means for withdrawing another individual fluid portion at a location between the outlet denoted by n=1 and the inlet means of (b).
An alternative embodiment of the invention relates to an apparatus for attenuating the amplitude of a time-variant property of a fluid which comprises an enclosed volume, wherein the enclosed volume includes
(a) a mixing zone having one or more inlets and one or more outlets, with the provisos that (1) the number of inlets and the number of outlets cannot both equal one, and (2) the number of inlets is at least three, or the number of outlets is at least three, or the number of inlets is at least three and the number of outlets is at least three;
(b) piping means for conducting the fluid to the one or more inlets of the mixing zone; and
(c) piping means for withdrawing fluid from the mixing zone through the one or more outlets and providing an outlet fluid stream.
In this embodiment, the distance between adjacent outlets decreases as the distance of each outlet from a given inlet increases and (1) the distance between at least one pair of adjacent inlets is different than the distance between another pair of adjacent inlets, or (2) the distance between at least one pair of adjacent outlets is different than the distance between another pair of adjacent outlets, or (3) the distance between at least one pair of adjacent inlets is different than the distance between another pair of adjacent inlets and the distance between at least one pair of adjacent outlets is different than the distance between another pair of adjacent outlets.